1. Field of the Invention
The present invention relates to a light emitting diode (LED) driver circuit. More particularly, the present invention relates to a circuit for controlling the power switch of an LED driver circuit by pulse width modulation (PWM).
2. Description of the Related Art
FIG. 1 is a schematic diagram showing a conventional LED driver circuit 100. The LED driver 100 consists of a boost DC/DC converter 130 and its load 120. The load 120 includes several strings of LEDs 104 and a current mirror 103 coupled to the LED strings. The boost converter 130 includes an inductor L1, a switch Q1, a PWM controller 101, a diode D1, resistors R11 and R12, and a capacitor C1. The inductor L1 has one end coupled to receive the input voltage Vin1 and the other end coupled to one end of the switch Q1. The anode of the diode D1 is coupled to the same end of the switch Q1. The capacitor C1 has one end coupled to the cathode of the diode D1 and the other end coupled to receive a ground voltage. The PWM controller 101 controls the switch Q1 through the PWM signal PS1. When the switch Q1 is turned on, the current of the inductor L1 increases. When the switch Q1 is turned off, the energy accumulated in the inductor L1 is transferred to the capacitor C1 to maintain a driving voltage Vout1 for the LEDs 104. The resistors R11 and R12 constitute a voltage divider and provide a feedback voltage FB1 to the PWM controller 101. The PWM controller 101 determines the duty cycle of the PWM signal PS1 according to the feedback voltage FB1.
The driving current determines the color of the light emitted from an LED. The brightness of an LED has to be controlled by the duty cycle of its driving current. For example, when the duty cycle is 50%, the brightness is 50% of the maximum level. In the driver circuit 100, the current mirror 103 provides driving current through the LEDs 104 when the dimming control signal DCS1 is asserted and cuts off the driving current when the dimming control signal DCS1 is de-asserted. Therefore the duty cycle of the dimming control signal DCS1 is the duty cycle of the driving current of the LEDs 104.
The switch Q1 is a power metal oxide semiconductor field effect transistor (power MOSFET). Power MOSFET is a large external component. Every time when a power MOSFET is turned on or turned off, it consumes considerable power due to gate charging or discharging. The PWM controller 101 outputs the PWM signal PS1 constantly. The power switch Q1 keeps switching even when the dimming control signal DCS1 is de-asserted, resulting in power waste and low efficiency. This problem is especially severe when the duty cycle of the dimming control signal DCS1 is low. Moreover, this unnecessary switching tends to overcharge the capacitor C1 and enlarge the ripple of the driving voltage Vout1.